Risk Management and Derivative Instruments - Interest Rate Swap Open Positions (Detail)	9 Months Ended
	Sep. 30, 2019 USD ($)
Interest Rate Swap Open Position Remaining 2019 [Member]
Derivative [Line Items]
Average Monthly Notional (in thousands)	$ 125,000,000
Weighted-average fixed rate	1.612%
Floating rate	1 Month LIBOR
Interest Rate Swap Open Position 2020 [Member]
Derivative [Line Items]
Average Monthly Notional (in thousands)	$ 125,000,000
Weighted-average fixed rate	1.612%
Floating rate	1 Month LIBOR
Interest Rate Swap Open Position 2021 [Member]
Derivative [Line Items]
Average Monthly Notional (in thousands)	$ 125,000,000
Weighted-average fixed rate	1.612%
Floating rate	1 Month LIBOR
Interest Rate Swap Open Position 2022 [Member]
Derivative [Line Items]
Average Monthly Notional (in thousands)	$ 75,000,000
Weighted-average fixed rate	1.281%
Floating rate	1 Month LIBOR